The present invention relates to pressure control valves and, more particularly, to a pressure control valve used in the hydraulic system of construction equipment such as a hydraulic excavator or the like.
Various pressure control valves are used in the hydraulic system of a hydraulic machine such as a hydraulic excavator or the like to protect circuits. For instance, a hydraulic system disclosed in U.S. Pat. No. 3,976,097 comprises a hydraulic circuit in which a hydraulic pump and a plurality of actuators are connected to each other through their respective directional control valves. In the hydraulic circuit, a relief valve and an unload valve are arranged in a delivery line of the hydraulic pump as pressure control means. The relief valve restricts maximum pressure of the delivery line to a value equal to a setting value (relief pressure) of a spring, to protect the circuit. The unload valve is operative in response to differential pressure between the delivery pressure of the hydraulic pump and maximum load pressure of the plurality of actuators. The pump delivery pressure is controlled such tha the differential pressure is retained to a setting value (unload differential pressure) of a spring. Thus, energy loss is reduced.
Conventionally, however, the following problems arise. Specifically, the relief valve and the unload valve are constructed as separate valve elements. If such pressure control valve is used in the hydraulic system, the number of parts of the hydraulic circuit increases, making the circuit arrangement complicated. Thus, the hydraulic system increases in cost.
On a other hand, although not the pressure control valve, U.S. Pat. No. 4,535,809 discloses a flow control valve which consists of a combination of a main valve element of seat type and a pilot valve element. That is, the flow control valve is constructed as follows. Specifically, a control chamber is located at a rearward face of the main valve element of seat type for biasing the main valve element in a valve closing direction. The control chamber communicates with an inlet of the main valve element through a variable restrictor which is variable in opening in response to displacement of the main valve element. The control chamber communicates with an outlet of the main valve element through a pilot passage. A pilot valve element is arranged in the pilot passage. Displacement of the main valve element is proportionally controlled in response to displacement of the pilot valve.
It is an object of the invention to provide a pressure control valve having a combined function of a relied valve and an unload valve with a compact valve construction, and capable of simplifying a circuit arrangement of a hydraulic system.